Gels are three-dimensional macromolecular networks that contain a large fraction of solvent within their structure and do not dissolve. When the trapped solvent is water, the gels are termed “hydrogels”. Hydrogels exhibit high water content and are soft and pliable. These properties are similar to natural tissue, and therefore hydrogels are extremely biocompatible and are particularly useful in biomedical and pharmaceutical applications.
Hydrogels can be responsive to a variety of external, environmental conditions. A unique physical property of some hydrogel systems is reversible volume changes with varying pH and temperature. This unique property and other characteristics are thoroughly detailed in the scientific reference articles cited above.
Some diversified uses of responsive gels include solute/solvent separations, biomedical tissue applications, controlled drug delivery, sensors and devices, and in NMR contrast agents. These applications are disclosed in U.S. Pat. Nos. 4,555,344, 4,912,032, 5,062,841, 5,976,648 and 5,532,006, respectively.
Polymer gels can be formed by the free radical polymerization of monomers in the presence of a reactive crosslinking agent and a solvent. They can be made either in bulk or in nano- or micro-particle form. The bulk gels are easy to handle, but usually have very slow swelling rates and amorphous structures arising from randomly crosslinked polymer chains. In contrast, gel nanoparticles react quickly to an external stimulus, have organized local structure, but suffer from practical size limitations.
Responsive polymer gels can be made by the co-polymerization of two different monomers, by producing interpenetrating polymer networks or by creating networks with microporous structures. These processes are disclosed in U.S. Pat. Nos. 4,732,930, 5,403,893, and 6,030,442, respectively. Finally, a microparticle composition and its method of use in drug delivery and diagnostic applications have also been disclosed in U.S. Pat. No. 5,654,006.
Hydrogels usually consist of randomly crosslinked polymer chains and contain a large amount of water occupying interstitial spaces of the network, resulting in amorphous structures. Without the addition of a coloring agent or opacifier, hydrogels are clear and colorless when they are fully swollen in water. To create colors in hydrogel systems, there are two major approaches in the prior art as disclosed in U.S. Pat. Nos. 6,165,389, 6,014,246 and 6,187,599. The first is to form a poly(N-isopropylacrylamide) (P-NIPA) crystalline colloidal fluid in an aqueous media and contain it in a glass cell. The second is to embed a crystalline colloidal array of polystyrene polymer solid spheres in a P-NIPA hydrogel. Both approaches have utilized the unique temperature-responsive property of the P-NIPA, but each has its own limitations. The first material is a colloidal fluid: its crystal structure can be easily destroyed by a small mechanical vibration. The second approach to make colored hydrogels requires the introduction of non-hydrogel solid spheres (polystyrene) as light-diffracting materials.
The concept for synthesizing crystal hydrogels based on crosslinking gel nanoparticles was first alluded to in provisional patent application Ser. No. 60/336,259. The nanoparticle networks as described in that patent application exhibit either a uniform color due to a short-range ordered structure or are colorless due to a randomly ordered structure.
The primary scope of this invention relates to environmentally responsive hydrogel nanoparticle networks that exhibit crystalline structures, are opalescent in appearance, are stable under mechanical vibration and temperature fluctuations, and consist of only hydrogel materials without other embedded solid polymer spheres.